Color shifting agents and method of using same



Dec. 18, 1951 F. 6. SMITH 5 9 COLOR SHIFTI'NG AGENTS AND METHOD OF USINGSAME Filed Feb. 27, 1948 2 SHEETS-Sl-EET 1 REFLECTION FACTOR WAVELENGTHm Mxnumcnons mvnrroa 55. SMITH Dec. 18, 1951 F. ca. SMITH 2,579,020

COLOR SHIFTING AGENTS AND um'non OF usmc SAME Filed Feb. 21, 1948 2sm'rs-sm'r 2 REFLECTION FACTOR,

ace 5 5' 400 500 600 700 750 WAVELENGTH m MILLIMICROHS JNVENTOR F.G.SMITH Patented Dec. 18, 1951 OFFICE COLOR SHIFTING AGENTS AND METHOD OFUSING SAME Frederick Gordon Smith, Toronto, Ontario, Canada ApplicationFebruary 27, 1948, Serial No. 11,488

14 Claims.

. This invention relates to color shifting agents for zinc oxidepigments and method of preparation of color shifted pigments.

This is a continuation-in-part application of my application Serial No.606,707, filed July 23, 1945, now abandoned.

Prior pigments are comprised of a neutral or colorless material forminga base or matrix which incorporates or has added thereto, a coloringmedium or material. Zinc oxide, magnesium oxide and silicon dioxideappear to be the most common matric materials and nowhere is itsuggested thatthese materials act as other than neutral substances whichgive body to the pigments and also serve to dilute the coloring efiectof the coloring medium or mediums.

The color of a pigment may be modified according to prior teaching bychanging the degree of fineness, of the grind, but in such instance, thecolor of the pigment is basically the same in the particular series.Thus, an orange pigment comprised of 99% zinc oxide and 1% manganeseoxide maybe altered toward yellow by increasing the fineness of thegrind. According to prior knowledge, magnesium oxide is recognized onlyas a neutral white body material since nowhere is it described as actingin any other capacity. Its action solely as a white body materialappears to have been taken for granted and is an accepted fact in theart.

The manufacturer of pigments must in many instances combine two pigmentsof a different color in order to obtain a color between the twopigments. For instance, it is necessary to prepare blue-greens in thismanner by mixing a green pigment and a blue-violet pigment in properproportion. When the spectral reflection curve of the final color orpigment mixture is compared with the spectral reflection curves of theoriginal pigments, it will be found that the curve of the final colorextends over the full wave length range of the two original colors onthe wave length base and that the dominant color derived from a hump onthe curve is modified in its effect by other humps being the modifiedremaining spectral reflection peaks of the original pigment curves. Itis admitted that by careful selection, a purely mechanical mixture oftwo pigments can result in a final color mixture which has good colordensity, but in many instances mixtures of pigments result in muddycolors where the spectral response curve has only a gentle hump ratherthan a definite peak.

In order that a compound may exhibit a color it must be transparent andselectively absorb some of the wave lengths of light passing through it.Among the inorganic colored compounds there are two types of suchselective absorption. One is the absorption on the long wave length sideof the principal absorption band. As this band is made to move into thevisible range, it appears first in the violet side of the spectrum andmay be made to move from the violet end to the red end by making thecompound more metallic in nature. The other type of absorption is thatdue to impurities or atomic groupings which form an absorption band onthe long wave length side of the principal absorption band. Suchabsorption bands are narrower than the principal absorption band and mayabsorb a small range of wave lengths and allow transmission ofwavelengths both longer and shorter than this range.

In accordance with the present invention particular regard is had to theimpurity type of absorption band and in this connection, I modify,change or shift the color absorption characteristics of a pigment bychanging the inter-atomic spacings and/or electronic configurationsaround constituent atoms. In the practical sense, however, the onlymaterial to which I have so far applied this new theory is zinc oxide inthe form of a solid solution and which includes a coloring material. Ihave tried titanium oxides and other metal oxides as a colorlessmaterial to which a coloring material has been added in a solid solutionwithout apparent success. It is peculiar that zinc oxide affords theonly material so far discovered or practically contemplated as amaterial subject to the mechanisms of the invention taught herein.

Although it is reasonable to assume that any element could enter into azinc oxide structure which includes a coloring material in solidsolution, only certain elements have given any appreciable result orshifting effect which is apparent to the human eye. Thus, in accordancewith the invention the zinc oxide and coloring material mixture in theform of a solid solution is subjected to the influence of what I termherein a color shifting agent which acts to shift the color responsecurve or the absorption band due to the impurity or coloring material inthe pigment. The color shifting agent, however, so far as is known, onlyenters the zinc positions of the atomic structure and only a certainlimited group of elements appear to give results which are useful to anydegree for pigment purposes.

The understanding of the mechanisms involved in employing a colorshifting agent in conjunction with a conventional zinc oxide typepigment to provide a color shift requires an appreciation of thestructure identified by the phrase solid solution." Reference should,therefore, be made to Introduction to Crystal Chemistry," R. C. Evans,Cambridge University Press 1939, chapt. 5, Alloy Systems, and chapters 6and '7.

Briefly, when pure zinc oxide, or any compound of zinc which decomposesthe zinc oxide on heating, is heated to a very high temperature, itcrystallizes to small, transparent, uncolored hexagonal crystals. Thisis well-known in the art. Zinc oxide has the property of holding arelatively large amount of many elements other than zinc in stable solidsolution and in this respect, is different from other materials such astitanium or other body materials ordinarily used in pi ments. It is thisproperty which makes zinc adaptable in the practical sense to the methodof color shifting according to the present invention.

The most convenient method for dissolving such elements in zinc oxide isto heat amorphous zinc oxide mixed with a small amount of anothermetallic oxide (coloring material) to a high temperature in well-knownmanner to crystallize the zinc oxide and at the same time incorporatethe impurity oxide within the crystal structure of the zinc oxide. Thus,cobalt will confer a saturated green color to zinc oxide and manganese asomewhat less saturated orange color. In accordance with the invention,however, the inclusion of a further constituent, that is to say, a colorshifting agent in solid solution will shift the color so that a newcolor will be obtained different from that expected to be derivedaccording to prior teachings.

Although as previously described, it is feasible that in the theoreticalsense any element might act as a color shifting agent, in the practicalsense I have found only two which are commercially satisfactory butwhich are satisfactory to a high degree. Further, it is fortunate thatone material will serve to shift the color response of a pigment of thetype described toward the violet end of the spectrum whereas the otherpreferred agent found shifts toward the red end of the spectrum and ineither case, the amount of color shift is proportional to the amount ofshifting agent ingredient held in solid solution.

Thus I have found that magnesium oxide in solid solution accomplishes ashift of the ordi" narily expected color of the zinc oxide body andcoloring material to a new place on the wave length base of the spectralresponse curve without effecting any substantial alteration in the formof the curve but shifting the same toward the violet end of thespectrum. Cadmium oxide in solid solution effects a shift toward the redend of the spectrum.

Materials which will given an appreciable color shift which is apparentto the human eye are found in group IIB of the periodic table butmercury and radium are not practical for use and are excepted. Thesematerials in this group are of a colorless type and I employ them in theoxide form.

I have also found that of the colored type of materials which also havea color shifting property to a degree which may be apparent to the humaneye, that iron, nickel and others of group VIII of the period table maybe selected from. It is particularly recommended, however, that coloredshifting agents be avoided since it is most difficult to predict theresults which may be obtained and in any case my experiments have shownthat the best results can be obtained from the shifting agents of thenon-colored type. For other reasons such as considerations of chemicalactivity, permanency and other factors, I prefer to use and recommendmagnesium oxide and cadmium oxide as color shifting agents and which areof the colorless type.

To illustrate precisely What occurs and to demonstrate the usefulness ofa color shifting agent, reference shall be made to the accompanyingdrawings wherein:

Figure 1 is the reflection factor versus wave length in millimicronsobtained by spectrophotometric tests for the visible region excludingspecular reflection, these curves being traced from actual curvesobtained by test and showing the shifting of a red pigment of the zincoxide type, toward the violet end of the spectrum by the use of onecolor shifting agent and toward the red end of the spectrum by the useof a different color shifting agent.

Figure 2 is a similar representation of the spectral reflection factorcurves for a green pigment which is shifted to a blue-green by the useof a color shifting agent.

Referring to the drawings:

Figure 1 shows the spectral reflection curve in bold line for a redpigment comprised of 15% manganese oxide with the remainder zinc oxide.The curve Y is the spectral response curve of a new pigment formed byusing magnesium oxide as a color shifting agent up to a value ofapproximately 20%, the color being yellow. The curve DR is the spectralreflection curve of a pigment formed by using cadmium oxide in theconcentration of 30% by weight, the color being dark red-violet. It willbe noticed that magnesium oxide shifts the color toward the violet endof the spectrum whereas cadmium oxide shifts the color toward the redend of the spec trum. In both instances, the form of the curve B issubstantially preserved, although it will be noted that the magnitude ofthe reflection value may be modified with an increase attendant in theuse of magnesium oxide.

In Figure 2, a green pigment represented by the curve G in bold linemade from a zinc oxide and cobalt oxide has been shifted by using thecolor shifting agent magnesium oxide to give the curve BG. The pigment Gis green Whereas the resulting color in the shifted pigment BG isbluegreen. The lines in and I! placed on this figure illustrate thedegree of shift accomplished. The form of the curve G again. has notbeen substan'' tially altered when subjected to the shifting mechanismsince the curve BG simulates the various humps and indentations of theoriginal curve G. It will be noted however, that the spectral reflectionvalue has been increased as compared with the peaks found in theoriginal curve G. The blue-green obtained is extremely clear andbrilliant to the human eye as compared with blue-greens prepared byprior art methods.

Although I have illustrated the simple shifting of the spectral responsecurve of a pigment by using a single color shifting agent, it will beapparent that the operations may be multiplied, where for instance,double color shift is at-- tempted. Thus, using an orange pigment ofzinc oxide type having 1% MnO and a green pigment green using the samepercentage of this color:

- shifting agent. This would be a single color shift for each pigment.Now if an olive-geenpigment tained will begin to grey oil? toward agreyish blue-green in the present example. I specify therefore, that thelimits defining the addition of a color shifting agent to a zinc oxidepigment is in each case the limit of solid solution of the colorshifting agent in hexagonal zinc oxide at the temperature of firing. Inapplying the method of shifting a color response curve of a pigmentaccording to the invention as taught herein, conventional and well-knownpractice was utilized in the combining of the ingredients including theselection of temperature of firing. Obviously, the temperature of firingwill vary Coloring Metallic Color Shifting Oxide (Approx- -Agent(Approximate Percentimate Percent- Oolor ages) ages) g 000 MnO MgO cablg (shifted red e 2 iiir't'e'i b'rhii Chrome-Yellow 1 Lemon-Yellow 1(Shifted chrome-yellow) Willow Green- M 1.. (double shift using 000Green 1 5 Blue- Green l is (shifted green) To prepare the colors betweengreen and yellow I prefer to use both cobalt and manganese with thecolor shifting agent magnesium .oxide as more saturated colors in theorange to red range,

than those previously obtainable with manganese alone. The addition ofmagnesium moves the yellow color due to manganese in the other directiontoward the violet end of the visible spectrum, the limiting color beinga saturated lemon-yellow when the maximum amount of magnesium oxide isin solid solution.

The apparent advantages of the color shifted colored zinc oxide forpigment purposes are the following: (1) A complete range of colors fromblue-green to orange-red, where any one color is composed of only onecompound and not a mixture of compounds, is provided; (2) All of 'thepigments are stable chemically and will withstand a very hightemperature; (3) The color of the pigments is permanent under normalconditions of weathering; (4) The pigments, over the entire range,transmit and refract the near infra-red radiation so that paintsprepared from them have a high infra-red reflectance, making them of usefor purposes such as simulating vegetation for camouflage.

' There are limits to the concentration of either cadmium oxide ormagnesium oxide where such may serve in the capacity of color shiftingagents. Thus, in a pigment comprised of 98% zinc oxide and 2% cobaltoxide, the green color may be shifted progressively toward blue-green bythe addition of magnesium oxide up to about by weight. After this pointhas been reached some pink crystals of cubic magnesium oxide will becolored by the cobalt oxide, resulting in a purplish-pink additionalpigment, the particles of which are of a color well-known by the termcobalt pink. Thus, if the shifting mechanism is extended beyondpractical limits the color obwidely as is well-known for differentpigments but no departure from welleknown practice in this regard iscontemplated. Obviously, the limits of the addition of eithermagnesiumoxide or cadmium oxide is determined to a degree by the temperature offiring and the time period of firinglat the chosen temperature. Thesefactors are dictated more by commercial requirements than the effectsproduced in the final pigment as is well-known. At a temperature of1200* C., safe maximums are 20% for magnesium oxide and 30% for cadmiumoxide, by weight of the compound.

I propose to use the shifted zinc oxide pigments described aboveinpaints in an oil vehicle for decorative and artistic purposes inso-called water colors for artistic purposes, in a water vehicle fordecorative purposes, in plastic and other vehicles for coloringpurposes, and as the coloring medium in decorated ceramic articles.

What I claim as my invention is:

1. A method of shifting the spectral response curve of a green pigmentcomposed of zinc oxide crystallized in the hexagonal system and havingcolouring materials of cobalt oxide present in the order of about 1% byweight of the compound and manganese oxide present in the order of about/2% by weight of the compound, contained within the crystal structure ofthe latter; and comprising the steps in combination of heating saidcompound to about 1200 degrees centigrade and adding a color shiftingagent of magnesium oxide in a concentration of about 20% by weight ofthe compound to absorb the same in solid solution within the crystalstructure of said pigment to providea colour shift in the originalpigment fromgreen to blue-green.

2. In a pigment compound which includes a base mixture of zinc oxidecrystallized in the hexagonal system and a composite colouring materialof cobalt oxide present in a concentration of about 1% by weight of thecompound and manganese oxide present in a concentration of about /2% byweight of the compound, contained within the crystal structure of thecompound in the form of a true solid solution; the combination therewithof a color shifting agent of magnesium oxide, the said magnesium oxideawaoa providing a shifting of-the original colour of the zinc oxide andcobalt oxide mixture toablue green when the amount of magnesiumoxide islarge and in a concentration of about 20% by weight of the compound, thesaidoriginal colour being green, and colours intermediate between greenand blue-green for lesser amounts of ma nesium oxide.

3. In a pigment compound composed of a base mixture of zinc oxidecrystallized in the hexagonal system and a composite colourin materialcomprised of both manganese oxide and cobalt oxide, in the form of atrue solid solution contained within the crystal structure of thecompound and having acolour series extending from green, when the cobaltoxide is in a concentration of about 1% by' weight of the compound andthe manganese oxide in a concentration of about /2% by weight of thecompound, to orange, when the cobalt oxide is present in a concentrationof about by weight of the compound and the manganese oxide is present ina concentration of about by Weight of the compound; the combinationtherewith of a color shifting agent of magnesium oxide present in aconcentration up to about by weight of the compound, the final colour ofthe compound including the color shifting agent being yellow-- orangewhen the ratio of the amount to cobalt oxide of manganese oxide is smalland the amount of magnesium oxide is small, the colour of the compoundbeing blue-green when the ratio of the amount of cobalt oxide tomanganese oxide is large and the amount of magnesium oxide is about 20%by weight of the compound, and intermediate colours such as green,yellowgreen, lemon-yellow, and yellow, when the ratio of the amount ofcobalt tomanganese is between the said limits and the amount ofmagnesium is less that 20% by weight of the compound but not zero. I I r4. A method of forming a different coloured pigment from a zinc oxidepigment having. a colouring metal oxide within the crystal structure ofthe zinc oxide in solid solution and where- .in the different colouredpigment to be formed possesses, a coloursaturation substantially thesame or greater than that of the zinc oxidecolouring metal pigment, andcomprising the steps in combination of: mechanically mixing with thezinc oxide-colouring metal pigment 2% to about by weight'of a colourlesscolour shifting agent in finely divided form selected from the groupconsisting of the oxides of magnesium and cadmium, heating the mixtureto about 1200? 'centigrade to cause the colour shifting agent to enterthe zinc positions ofthe atomic structure of the zinc oxide and toenterinto solid solution therewith.

5. A pigment comprising in combination: zinc oxide, a colouring metaloxide disposed in solid solution within the crystal structure of thezinc oxide, and a colourless colour shifting agent selected from thegroup consisting of magnesium oxide and cadmium oxide present in anamount of between 2% and about 30% by weight of the zinc oxide andcolouring metal in solid solution in said pigment.

6. A pigment comprising in combination zinc oxide, a coloring metaloxide selected from the group consisting of cobalt oxide, manganeseoxide, and mixtures of cobalt oxide and manganese oxide, disposed insolid solution within the crystal structure of the zinc oxide, and acolorless color shifting agent selected from the group consisting ofmagnesium oxide and cadmium oxide present in an amount of between 2% andabout 30% by weight of the zinc oxide and coloring metal oxide insolidsolution in said pigment.

'7. A zinc oxide pigment in accordance with claim 6, in which thecoloring metal oxide is cobalt oxide.

8. A zinc oxide pigment in accordance with claim '7 in which the colorshifting agent is magnesium oxide in an amount from about 2. to about20% by weight.

9. A zinc oxide pigment in accordancewith claim '7 in which the colorshifting agent is cadmium oxide in an amount from about 2 to about 30%by weight.

10. A zinc oxide pigment in accordance with claim 6 in which thecoloring metal oxide is manganese oxide.

11. A zinc oxide pigment in accordance with c1aim.10 in which the colorshifting agent is mag.- nesium oxide in an amount from about 2 to about20% by weight.

12. A zinc oxide pigment in accordance with claim 10 in which the colorshifting agent is cadmium oxide in an amount from about 2 to about 30%by weight.

13. A zinc oxide pigment in accordance with claim 6 in which thecoloring metal oxide is a mixture of manganese oxide and cobalt oxide.

14. A zinc oxide pigment in accordance with claim 13 in which the colorshifting agent is magnesium oxide in an amount from about 2 to about 20%by weight.

FREDERICK GORDON SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Name Date Korinth et al Jan. 28, 1936 Korinth eta1 Jan. 19, 1937 FOREIGN PATENTS Country Date France Aug. 31, 1932 OTHERREFERENCES Handbook of Chemistry and. Physics, 22nd ed.. Chemical RubberPub. Co., Cleveland, Ohio, 1937, pg. 366, Nos. 4 and 5; pg. 408, No. 10.

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5. A PIGMENT COMPRISING IN COMBINATION: ZINC OXIDE, A COLOURING METALOXIDE DISPOSED IN SOLID SOLUTION WITHIN THE CRYSTAL STRUCTURE OF THEZINC OXIDE, AND A COLOURLESS COLOUR SHIFTING AGENT SELECTED FROM THEGROUP CONSISTING OF MAGNESIUM OXIDE AND CADMIUM OXIDE PRESENT IN ANAMOUNT OF BETWEEN 2% AND ABOUT 30% BY WEIGHT OF THE ZINC OXIDE ANDCOLOURING METAL IN SOLID SOLUTION IN SAID PIGMENT.